1. Field of the Invention
The present invention relates to a new and improved process for the preparation of alkyl- or arylphosphonothioic dihalides.
2. The Prior Art
Alkylphosphonothioic dihalides have been prepared in the prior art by reacting alkyl halides with phosphorus trihalides in the presence of aluminum chloride. The reaction proceeds at room temperature according to the formula set forth in Heuben-Weyl, Methoden der Organis Chenchemie, Volume 12, part 1 (1965) at page 396. ##EQU1## The Heuben-Weyl reference also notes that the reaction has been attempted in the absence of the aluminum chloride catalyst with little success. The alkylphosphonothioic dihalides are prepared by replacing the oxygen of the alkylphosphonic dihalide with sulfur as shown on page 553 of the Heuben-Weyl reference. The yields of the replacement reaction are limited to the yields obtained in the initial reaction forming the alkyl-phosphonic dihalide.
Phosphorus sulfides are known in the form of four well-characterized crystalline compounds, namely, P.sub.4 S.sub.1 o, P.sub.4 S.sub.7, P.sub.4 S.sub.5 and P.sub.4 S.sub.3. Of these, P.sub.4 S.sub.1 o and P.sub.4 S.sub.3 are produced commercially.
It is known that alkyl- or arylphosphonous dichlorides can be prepared from an alkyl or aryl halide, phosphorus trichloride and phosphorus at elevated temperatures in an autoclave and under autogenous pressure ("Synthesis of Alkyl- and Arylphosphorus Dichlorides and Dibromides"--Zhurnol Obshchei Khimii, Vol. 37 No. 4, pp. 890-892, Apr. 1967).
A method of preparing alkyl- or arylphosphonothioic dihalides according to the following reaction scheme: EQU 3RX + aPX.sub.3 + bP(S)X.sub.3 + (0.3a + 0.2b)P.sub.4 S.sub.1 o+ (0.8a + 1.2b)P .fwdarw. 3RP(S)X.sub.2 ( 2)
where a + b = 1, is described in U.S. Pat. No. 3,803,226. That method, however, has the disadvantage that yellow phosphorus must be handled to effect the reaction.